Chaging VideoLayers -> VideoLayer (#1591)

There was mixed used. It is a struct. So, it is a singular.
Change all the places I could find. There may be more, but can be
changed when spotted.

pkg/sfu/buffer/videolayer.go

@@ -11,12 +11,12 @@ const (
 )
 
 var (
-	InvalidLayers = VideoLayer{
+	InvalidLayer = VideoLayer{
 		Spatial:  InvalidLayerSpatial,
 		Temporal: InvalidLayerTemporal,
 	}
 
-	DefaultMaxLayers = VideoLayer{
+	DefaultMaxLayer = VideoLayer{
 		Spatial:  DefaultMaxLayerSpatial,
 		Temporal: DefaultMaxLayerTemporal,
 	}